1. Field of the Invention
This invention relates to an organic electroluminescent device whose durability has been improved.
2. Related Arts
Recently, in accordance with the diversification of information apparatuses, the demand for flat-type display devices has been growing which manage with less electric power and smaller space than cathode-ray tubes (CRT). Included in such flat-type displays are crystalline liquid display devices and plasma display devices, and drawing special attention among them is an electroluminescent (hereinafter referred to as EL) device which is a self-luminescent type and can provide clear display.
Here, the EL devices can be divided between inorganic and organic depending on the constituent materials, the former having already been put to practical use.
However, such an inorganic EL device needs to be driven with high voltage because its driving type is a so-called `collisional excitation type` wherein electrons accelerated by the impression of a high electrical field make the luminescence center luminesce through the collisional excitation. This construction brings about an increase in the cost of surrounding devices.
On the other hand, the organic EL device can be driven with low voltage because it exhibits so-called `injection type luminescence` wherein charges (holes injected from an electrode and electrons injected from another) are combined with each other inside emitting materials to produce luminescence. It has another advantage of being able to easily produce any desired emitting colors by altering the molecular structure of the organic compound. Hence, such organic EL devices are very hopeful as new display devices.
The organic EL devices generally have either two-layer or three-layer structure. The two-layer structure has either SH-A structure wherein an emitting layer and a hole transport layer are formed between a hole injection electrode and an electron injection electrode, or SH-B structure wherein an emitting layer and an electron transport layer are formed between these electrodes. The three-layer structure has DH structure wherein an emitting layer, a hole transport layer, and an electron transport layer are formed between these electrodes. Used for such hole injection electrodes are materials having a large work function such as gold and ITO (In-Sn oxide), while used for such electron injection electrodes are materials having a small work function such as Mg. All the layers comprise organic materials: the hole transport layer comprises a material having p-type semiconductor characteristics, the electron transport layer comprises a material having n-type semi-conductor characteristics, and the emitting layer comprises a material having n-type semi-conductor characteristics when used in the SH-A structure, a material having p-type semi-conductor characteristics when used in the SH-B structure, and a material having characteristics close to neutral when used in the DH structure. Every structure described above is based on the common principle that holes injected from a hole injection electrode and electrons injected from an electron injection electrode are combined on the boundary surface between an emitting layer and a hole (or electron) transport layer as well as inside the emitting layer to produce luminescence.
Although the organic EL devices, as mentioned before, have advantages of being driven with low voltage, producing any desirable emitting colors theoretically, and the like, they still have a disadvantage of being pretty short in their life. This is because very few of the compounds used for the organic emitting layer or the organic electron transport layer can retain stable luminescence with good film-forming properties. Therefore, if an EL device is produced with such a compound, undesired precipitates might be caused between adjacent layers laminated, or the compound itself might deteriorate.
As for the organic emitting layer, 8-quinolinol-aluminum complex is conventionally used as an emitting material having stable life with good film-forming properties. No equivalents of the 8-quinolinol aluminum are found so far, which are greatly desired.